A nuclear winter was a planet-wide alteration of the climate, usually as the result of a nuclear war or other major events.
Earth experienced nuclear winters in the 21st century after World War III. (TNG: "Encounter at Farpoint", "A Matter of Time")
When Terralysium's outermost ring destabilized in 2257, radioactive debris threatened to bombard the planet and trigger a nuclear winter. The threat was averted by the USS Discovery, which used the gravity of a dark matter asteroid to drag the debris away from the planet. (DIS: "New Eden")
In 2368, an asteroid impacted on Penthara IV, which threatened to cause a nuclear winter, but the USS Enterprise-D was able to prevent such a disaster. (TNG: "A Matter of Time")
The Vaadwaur homeworld suffered from a nuclear winter, after massive orbital bombardment, from 1484 until at least 2376. (VOY: "Dragon's Teeth")
Otrin's homeworld experienced a nuclear winter, due to a containment accident in its power grid, from the late-23rd century until 2378 when the USS Voyager neutralized the antimatter radiation. Harry Kim noted that it wouldn't have taken that many warheads to cause such an event. (VOY: "Friendship One")
Talos IV was said to be devastated by atomic war, though it is unclear if this event included a nuclear winter.